abc's john donvan has the story. >> reporter: hitting the fast food drive through for science. >> can i get five soft tacos? >> can i get two sausage burritos? >> can i get a number four? >> reporter: over and over and over again. it has been dave's life the past three months or so as has been eating the food. a precisely measured, 1,000 calories a day, extra meal daily, fast food only. how many calories? >> 770. >> reporter: why? this man is paying them to. dr. samuel klein is a researcher at washington university medical school in st. louis trying to understand why weight gain leads to diseases like diabetes and hypertension and how that relates to fat in the live and muscles. at some point research on rats alone just is not enough. somebody has to eat the food. this way -- >> it has to come to people. >> announcer: attention overweight volunteers. >> reporter: a radio ad offered a cash incentive up to $3,500 depending how long it would take. >> once i got to work i called right away. >> reporter: so did nurse dawn freeman. >> it probably took a month -- >> reporter: to get approved